Switching power supplies are widely used in electronic systems because they are more efficient and typically smaller and lighter than dissipative supplies of comparable output power. One class of such switching power supplies uses a fixed-frequency clock oscillator, having a substantially fixed frequency output with variable duty cycle PWM, whereby the output pulse width is modulated dependent on input to output voltage differential and load current. An error voltage responsive to the difference between the desired output voltage or current and the actual output voltage or current is generated and coupled to the PWM, so as to modify its duty cycle at the frequency of the clock oscillator in a manner which minimizes this error voltage.
One drawback to switching supplies is the electrical noise generated at the frequency of the clock oscillator, sometimes referred to as radio frequency interference (RFI) or electromagnetic interference (EMI). The current through the switching element in such a supply is typically switched from substantially fully on to fully off at the frequency of the clock oscillator, creating large circulating currents in circuit traces. These large currents at the PWM frequency may then radiate as RFI or couple to other circuit traces through capacitive or magnetic coupling. Known techniques, such as proper circuit trace layout and shielding, may reduce this electrical noise at the clock frequency, but may not fully suppress the undesired interference, especially when the clock frequency falls within the operating frequency range of the circuit being powered. An example might be the use of a switching power supply with a clock frequency of 1 MHz in a television system. If even a small amount of 1 MHz signal couples into the analog video signal, it may be visible in the displayed image as faint but annoying diagonal lines in the image. By spreading the spectrum of the PWM signal, the subjective impairment of such interference is often significantly reduced.
Known circuits and methods exist for spreading the spectrum of the PWM signal, typically employing a spreading oscillator at a frequency substantially lower than the clock oscillator operating frequency. The output of this spreading oscillator is coupled to a node within the clock oscillator so as to cause the frequency of the clock oscillator to change, dependent on the instantaneous amplitude of the spreading oscillator output. The spreading oscillator typically uses the time constant of a resistor and a capacitor to set its operating frequency, or alternatively may use other circuits such as a digital waveform generator comprising a lookup table and digital to analog converter.
An apparatus and method for generating a spreading signal without the need for an additional frequency-determining capacitor is desirable, especially when the PWM controller is an integrated circuit. Such an additional capacitor is typically too large to be integrated, and typically requires another pin on the integrated circuit to support it. A spreading oscillator which utilizes a capacitor already used by the PWM controller for another function is therefore desirable, and is an object of the present invention.